linux-kernel - [PATCH v1] mm/ksm: remove page_mapcount() usage in stable_tree

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Date: Tue, 16 Apr 2024 19:25:33 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alex Shi <alexs@...nel.org>
Subject: [PATCH v1] mm/ksm: remove page_mapcount() usage in stable_tree_search()

We want to limit the use of page_mapcount() to the places where it is
absolutely necessary.

If our folio has a stable node, it is a (small) KSM folio -- see
folio_stable_node(). Let's use folio_mapcount() in stable_tree_search()
instead, which results in no functional change.

The mapcount > 1 check is a bit confusing, because that's usually a check
for page sharing. Looks like the reason is that we are guaranteed to not
exceed ksm_max_page_sharing for the tree KSM folio when merging with
that. Let's update the documentation to make that clearer.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Alex Shi (tencent) <alexs@...nel.org>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/ksm.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 486c9974f8e20..159604ad47799 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1909,12 +1909,15 @@ static struct page *stable_tree_search(struct page *page)
 			if (page_node) {
 				VM_BUG_ON(page_node->head != &migrate_nodes);
 				/*
-				 * Test if the migrated page should be merged
-				 * into a stable node dup. If the mapcount is
-				 * 1 we can migrate it with another KSM page
-				 * without adding it to the chain.
+				 * If the mapcount of our migrated KSM folio is
+				 * at most 1, we can merge it with another
+				 * KSM folio where we know that we have space
+				 * for one more mapping without exceeding the
+				 * ksm_max_page_sharing limit: see
+				 * chain_prune(). This way, we can avoid adding
+				 * this stable node to the chain.
 				 */
-				if (page_mapcount(page) > 1)
+				if (folio_mapcount(folio) > 1)
 					goto chain_append;
 			}
 
-- 
2.44.0